Ultrasonic transducers are used to emit and detect ultrasonic waves in particular media of interest. Ultrasonic waves are used in various applications, including nondestructive evaluation, medical imaging, ranging, and signal processing (filters, delay lines, convolvers, etc.). Depending on the application, the ultrasonic waves need to be excited in and detected from either solids, liquids, or gases. Piezoelectric transducers work very well with solids. However, due to the acoustic impedance mismatch between piezoelectric ceramics and less dense media such as liquids and gases, piezoelectric devices are not the optimal performers in many applications. Capacitive transducers are a better alternative in airborne ultrasonic applications, and if they can be microfabricated, have distinct advantages in fluid immersion applications.
The fabrication of microfabricated ultrasonic transducers which operate on an electrostatic principle is described in co-pending patent application Ser. No. 08/327,210 filed Oct. 21, 1994 and a broadband version of such transducers is described in patent application Ser. No. 08/739,446 filed Oct. 29, 1996 (filed simultaneously herewith).
Applications where ultrasound is used in liquid media are currently very significant. From medical ultrasound to industrial cleaning, fluid coupled ultrasound is used in most economically important applications. Because of the fabrication advantages inherent in microfabricating and because of the increased transmission power and receive sensitivity of capacitive transducers, there is a need for microfabricated fluid immersion capacitive transducers.